Studies on the onset of brain peptide gene expression have shown for vasopressin (AVP), oxytocin (OT), luteinizing releasing hormone (LHRH), and pro-opiomelanocortin (POMC) that 1) these peptides are expressed well before their "classical" functions are operative in the animal, suggesting alternative (trophic) roles for these peptides during embryonic development, 2) AVP, OT, and POMC peptides are expressed post-migration, whereas LHRH is expressed prior to migration, 3) LHRH neurons appear to be generated in extra-neural loci and migrate into the brain to reach their mature positions, and 4) gene expression and post-translational modification mechanisms in peptidergic neurons appear to be independently regulated during development. Two in vitro tissue culture systems, dissociated and slice-explant organotypic cultures, have been developed, and allow for the long-term growth and maintenance of hypothalamic peptidergic neurons in vitro. Studies on the regulation of peptide gene expression, using in situ hybridization histochemistry have been successfully performed in primary cultures. The existence of two major subpopulations of corticotropic releasing hormone (CRH) neurons in the paraventricular nucleus (AVP+AVP-) has been documented by serial section electron microscopic immunocytochemistry. The roles of cholecystokinin, dynorphin, and CRH as copeptides in the neurohypophysis have been studied, and these peptides have been found to modulated AVP and OT secretion by different mechanisms. Gastrin-releasing peptide gene expression has been topographically mapped in the rat brain, and transgenic mice containing constructs of AVP and OT promoter regions linked to beta-galactosidase reporter genes have been produced.